Human T cells use CD1 and MR1 to recognize lipids and small molecules

CD1a-mediated immunity from a molecular perspective

Human CD1a is a non-polymorphic glycoprotein that presents lipid antigens to T cells. The most obvious role of CD1a is associated with its expression on Langerhans cells in epidermis, where it is involved in responses to pathogens. Antigen-specific T cells are believed to co-recognise CD1a presenting bacterial antigens such as species of lipopeptides from Mycobacterium tuberculosis. Further, human skin contains large amount of endogenous lipids that can activate distinct subsets of CD1a-restricted autoreactive T cells, mostly belonging to the αβ lineage, which are abundant in human blood and skin and are important for skin homeostasis in healthy individuals. CD1a and CD1a-restricted T cells have been linked to certain autoimmune conditions such as psoriasis, atopic dermatitis and contact hypersensitivity becoming a potential candidate for clinical interventions. A significant progress has been made in the last twenty years towards our understanding of the molecular processes that orchestrate CD1a-lipid binding, antigen presentation and mechanism of CD1a recognition by αβ and γδ T cells. This review summarises the recent developments within the field of CD1a-mediated immunity from a molecular perspective.

CD1a function in human skin disease

The high expression of CD1a on Langerhans cells in normal human skin suggests a central role for this lipid antigen presenting molecule in skin homeostasis and immunity. Although the lipid antigen presenting function of CD1a has been known for years, the physiological and pathological functions of the CD1a system in human skin remain incompletely understood. This review provides an overview of this active area of investigation, and discusses recent insights into the functions of CD1a, CD1a-restricted T cells, and lipid antigens in inflammatory and allergic skin disease. We include recent publications and work presented at the biennial CD1-MR1 EMBO workshop held in 2019 in Oxford, regarding lipids that increase and those that decrease T cell responses to CD1a.

Hollow multishell structures exercise temporal–spatial ordering and dynamic smart behaviour

A hollow multishell structure (HoMS) is an assembly of multiple shells with voids between the individual shells. Accessible through nanopores, these voids represent separate reaction environments in the same assembly, such that HoMSs have unique properties that are applicable to diverse fields. These applications have mostly exploited the large specific surface area, high loading capacity and/or buffering effect of HoMSs, benefiting the mass/energy transmission and effective surface area. In comparison, the temporal-spatial ordering of reactions, as well as the dynamic smart behaviour of HoMSs, have been less explored but are also emphasized in this Perspective. We first describe the synthesis of HoMSs and the thermodynamic and kinetic aspects of their formation. We then consider the composition and structural functionalization of each shell within a HoMS and then highlight how these enable applications based on temporal-spatial ordering and dynamic smart behaviour.

The lipid transfer protein Saposin B does not directly bind CD1d for lipid antigen loading

Background: Lipid antigens are presented on the surface of cells by the CD1 family of glycoproteins, which have structural and functional similarity to MHC class I molecules. The hydrophobic lipid antigens are embedded in membranes and inaccessible to the lumenal lipid-binding domain of CD1 molecules. Therefore, CD1 molecules require lipid transfer proteins for lipid loading and editing. CD1d is loaded with lipids in late endocytic compartments, and lipid transfer proteins of the saposin family have been shown to play a crucial role in this process. However, the mechanism by which saposins facilitate lipid binding to CD1 molecules is not known and is thought to involve transient interactions between protein components to ensure CD1-lipid complexes can be efficiently trafficked to the plasma membrane for antigen presentation. Of the four saposin proteins, the importance of Saposin B (SapB) for loading of CD1d is the most well-characterised. However, a direct interaction between CD1d and SapB has yet to be described. Methods: In order to determine how SapB might load lipids onto CD1d, we used purified, recombinant CD1d and SapB and carried out a series of highly sensitive binding assays to monitor direct interactions. We performed equilibrium binding analysis, chemical cross-linking and co-crystallisation experiments, under a range of different conditions. Results: We could not demonstrate a direct interaction between SapB and CD1d using any of these binding assays. Conclusions: This work strongly indicates that the role of SapB in lipid loading does not involve direct binding to CD1d. We discuss the implication of this for our understanding of lipid loading of CD1d and propose several factors that may influence this process.

Mycobacterium tuberculosis releases an antacid that remodels phagosomes

Mycobacterium tuberculosis (Mtb) is the world's most deadly pathogen. Unlike less virulent mycobacteria, Mtb produces 1-tuberculosinyladenosine (1-TbAd), an unusual terpene nucleoside of unknown function. In the present study 1-TbAd has been shown to be a naturally evolved phagolysosome disruptor. 1-TbAd is highly prevalent among patient-derived Mtb strains, where it is among the most abundant lipids produced. Synthesis of TbAd analogs and their testing in cells demonstrate that their biological action is dependent on lipid linkage to the 1-position of adenosine, which creates a strong conjugate base. Furthermore, C20 lipid moieties confer passage through membranes. 1-TbAd selectively accumulates in acidic compartments, where it neutralizes the pH and swells lysosomes, obliterating their multilamellar structure. During macrophage infection, a 1-TbAd biosynthesis gene (Rv3378c) confers marked phagosomal swelling and intraphagosomal inclusions, demonstrating an essential role in regulating the Mtb cellular microenvironment. Although macrophages kill intracellular bacteria through phagosome acidification, Mtb coats itself abundantly with antacid.

The lipid transfer protein Saposin B does not directly bind CD1d for lipid antigen loading

Background: Lipid antigens are presented on the surface of cells by the CD1 family of glycoproteins, which have structural and functional similarity to MHC class I molecules. The hydrophobic lipid antigens are embedded in membranes and inaccessible to the lumenal lipid-binding domain of CD1 molecules. Therefore, CD1 molecules require lipid transfer proteins for lipid loading and editing. CD1d is loaded with lipids in late endocytic compartments, and lipid transfer proteins of the saposin family have been shown to play a crucial role in this process. However, the mechanism by which saposins facilitate lipid binding to CD1 molecules is not known and is thought to involve transient interactions between protein components to ensure CD1-lipid complexes can be efficiently trafficked to the plasma membrane for antigen presentation. Of the four saposin proteins, the importance of Saposin B (SapB) for loading of CD1d is the most well-characterised. However, a direct interaction between CD1d and SapB has yet to be described. Methods: In order to determine how SapB might load lipids onto CD1d, we used purified, recombinant CD1d and SapB and carried out a series of highly sensitive binding assays to monitor direct interactions. We performed equilibrium binding analysis, chemical cross-linking and co-crystallisation experiments, under a range of different conditions. Results: We could not demonstrate a direct interaction between SapB and CD1d using any of these binding assays. Conclusions: This work establishes comprehensively that the role of SapB in lipid loading does not involve direct binding to CD1d. We discuss the implication of this for our understanding of lipid loading of CD1d and propose several factors that may influence this process.

Molecular cloning and characterization of the pig MHC class Ⅰ-related MR1 gene

Major histocompatibility complex (MHC) class Ⅰ-related protein 1 (MR1), the most highly conserved MHC class Ⅰ molecule among mammals, is the restricting molecule for mucosal-associated invariant T (MAIT) cells. MAIT cells, a novel subset of T cells, play important roles in modulating the immune responses to infectious and non-infectious diseases, and recognize antigens in the context of MR1. MR1 has been identified in many species, including human, mouse, sheep, and cow. Here, we cloned and characterized pig (Sus scrofa) MR1 (pMR1) transcripts, including five unique splice variants, from pig peripheral blood mononuclear cell cDNA. We also examined the tissue distribution of pMR1 and confirmed reactivity of pMR1 using a MR1 specific monoclonal antibody 26.5, demonstrating that the pMR1 gene was expressed in all tested tissues. Finally, we predicted the pMR1 3D structure and analyzed the docking mode of the MR1-5-OP-RU complex, finding that the docking mode of pMR1 with 5-OP-RU is similar to human MR1 docking. Collectively, this description of pMR1 adds to our understanding of the evolution of MHC molecules, and provides a theoretical basis for the subsequent study of pig MAIT cells.

Innate and Adaptive Immune Responses during Listeria monocytogenes Infection

It could be argued that we understand the immune response to infection with Listeria monocytogenes better than the immunity elicited by any other bacteria. L. monocytogenes are Gram-positive bacteria that are genetically tractable and easy to cultivate in vitro, and the mouse model of intravenous (i.v.) inoculation is highly reproducible. For these reasons, immunologists frequently use the mouse model of systemic listeriosis to dissect the mechanisms used by mammalian hosts to recognize and respond to infection. This article provides an overview of what we have learned over the past few decades and is divided into three sections: "Innate Immunity" describes how the host initially detects the presence of L. monocytogenes and characterizes the soluble and cellular responses that occur during the first few days postinfection; "Adaptive Immunity" discusses the exquisitely specific T cell response that mediates complete clearance of infection and immunological memory; "Use of Attenuated Listeria as a Vaccine Vector" highlights the ways that investigators have exploited our extensive knowledge of anti-Listeria immunity to develop cancer therapeutics.

Discovery of Salmonella trehalose phospholipids reveals functional convergence with mycobacteria

Salmonella species are among the world's most prevalent pathogens. Because the cell wall interfaces with the host, we designed a lipidomics approach to reveal pathogen-specific cell wall compounds. Among the molecules differentially expressed between Salmonella Paratyphi and S. Typhi, we focused on lipids that are enriched in S. Typhi, because it causes typhoid fever. We discovered a previously unknown family of trehalose phospholipids, 6,6'-diphosphatidyltrehalose (diPT) and 6-phosphatidyltrehalose (PT). Cardiolipin synthase B (ClsB) is essential for PT and diPT but not for cardiolipin biosynthesis. Chemotyping outperformed clsB homology analysis in evaluating synthesis of diPT. DiPT is restricted to a subset of Gram-negative bacteria: large amounts are produced by S. Typhi, lower amounts by other pathogens, and variable amounts by Escherichia coli strains. DiPT activates Mincle, a macrophage activating receptor that also recognizes mycobacterial cord factor (6,6'-trehalose dimycolate). Thus, Gram-negative bacteria show convergent function with mycobacteria. Overall, we discovered a previously unknown immunostimulant that is selectively expressed among medically important bacterial species.

Phospholipid analysis in sera of horses with allergic dermatitis and in matched healthy controls

Background

Lipids have become an important target for searching new biomarkers typical of different autoimmune and allergic diseases. The most common allergic dermatitis of the horse is related to stings of insects and is known as insect bite hypersensitivity (IBH) or summer eczema, referring to its recurrence during the summer months. This intense pruritus has certain similarities with atopic dermatitis of humans. The treatment of IBH is difficult and therefore new strategies for therapy are needed. Autoserum therapy based on the use of serum phospholipids has recently been introduced for horses. So far, serum lipids relating to these allergic disorders have been poorly determined. The main aim of this study was to analyse phospholipid profiles in the sera of horses with allergic dermatitis and in their healthy controls and to further assess whether these lipid profiles change according to the clinical status after therapy.

Methods

Sera were collected from 10 horses with allergic dermatitis and from 10 matched healthy controls both before and 4 weeks after the therapy of the affected horses. Eczema horses were treated with an autogenous preparation made from a horse’s own serum and used for oral medication. Samples were analysed for their phospholipid content by liquid chromatography coupled to a triple-quadrupole mass spectrometer (LC-MS). Data of phospholipid concentrations between the groups and over the time were analysed by using the Friedman test. Correlations between the change of concentrations and the clinical status were assessed by Spearman’s rank correlation test.

Results

The major phospholipid classes detected were phosphatidylcholine (PC), sphingomyelin (SM), phosphatidylinositol (PI) and phosphatidylethanolamine (PE). Eczema horses had significantly lower total concentrations of PC (p < 0.0001) and SM (p = 0.0115) than their healthy controls. After a 4-week therapy, no significant differences were found between the groups. Changes in SM concentrations correlated significantly with alterations in clinical signs (p = 0.0047).

Conclusions

Horses with allergic dermatitis have an altered phospholipid profile in their sera as compared with healthy horses and these profiles seem to change according to their clinical status. Sphingomyelin seems to have an active role in the course of equine insect bite hypersensitivity.

Elevated and cross-responsive CD1a-reactive T cells in bee and wasp venom allergic individuals

The role of CD1a-reactive T cells in human allergic disease is unknown. We have previously shown that circulating CD1a-reactive T cells recognize neolipid antigens generated by bee and wasp venom phospholipase, and here tested the hypothesis that venom-responsive CD1a-reactive T cells associate with venom allergy. Circulating T cells from bee and wasp venom allergic individuals, before and during immunotherapy, were exposed to CD1a-transfected K562 cells in the presence of wasp or bee venom. T-cell response was evaluated based on IFNγ, GM-CSF, and IL-13 cytokine production. Venom allergic individuals showed significantly higher frequencies of IFN-γ, GM-CSF, and IL-13 producing CD1a-reactive T cells responsive to venom and venom-derived phospholipase than healthy individuals. Venom-responsive CD1a-reactive T cells were cross-responsive between wasp and bee suggesting shared pathways of allergenicity. Frequencies of CD1a-reactive T cells were initially induced during subcutaneous immunotherapy, peaking by weeks 5, but then reduced despite escalation of antigen dose. Our current understanding of venom allergy and immunotherapy is largely based on peptide and protein-specific T cell and antibody responses. Here, we show that lipid antigens and CD1a-reactive T cells associate with the allergic response. These data have implications for mechanisms of allergy and approaches to immunotherapy.

Donor Unrestricted T Cells: A Shared Human T Cell Response

The now-famous term "restriction" derived from experiments in which T cells from Donor A failed to recognize Ags presented by cells from Donor B. Restriction results from interdonor variation in MHC genes. Donor restriction dominates immunologists' thinking about the T cell response because it governs organ transplantation and hinders the discovery of disease-associated Ags. However, other T cells can be considered "donor unrestricted" because their targets, CD1a, CD1b, CD1c, CD1d, or MR1, are expressed in a similar form among all humans. A striking feature of donor unrestricted T cells is the expression of invariant TCRs with nearly species-wide distribution. In this article, we review new evidence that donor unrestricted T cells are common in humans. NKT cells, mucosa-associated invariant T cells, and germline-encoded mycolyl-reactive T cells operate outside of the familiar principles of the MHC system, providing a broader picture of T cell function and new opportunities for therapy.

Lipid and small-molecule display by CD1 and MR1

The antigen-presenting molecules CD1 and MHC class I-related protein (MR1) display lipids and small molecules to T cells. The antigen display platforms in the four CD1 proteins are laterally asymmetrical, so that the T cell receptor (TCR)-binding surfaces are comprised of roofs and portals, rather than the long grooves seen in the MHC antigen-presenting molecules. TCRs can bind CD1 proteins with left-sided or right-sided footprints, creating unexpected modes of antigen recognition. The use of tetramers of human CD1a, CD1b, CD1c or MR1 proteins now allows detailed analysis of the human T cell repertoire, which has revealed new invariant TCRs that bind CD1b molecules and are different from those that define natural killer T cells and mucosal-associated invariant T cells.

Early innate responses to pathogens: pattern recognition by unconventional human T-cells

Although typically viewed as a feature of innate immune responses, microbial pattern recognition is increasingly acknowledged as a function of particular cells nominally categorized within the adaptive immune system. Groundbreaking research over the past three years has shown how unconventional human T-cells carrying invariant or semi-invariant TCRs that are not restricted by classical MHC molecules sense microbial compounds via entirely novel antigen presenting pathways. This review will focus on the innate-like recognition of non-self metabolites by Vγ9/Vδ2 T-cells, mucosal-associated invariant T (MAIT) cells and germline-encoded mycolyl-reactive (GEM) T-cells, with an emphasis on early immune responses in acute infection.

Understanding FRET as a research tool for cellular studies

Communication of molecular species through dynamic association and/or dissociation at various cellular sites governs biological functions. Understanding these physiological processes require delineation of molecular events occurring at the level of individual complexes in a living cell. Among the few non-invasive approaches with nanometer resolution are methods based on Förster Resonance Energy Transfer (FRET). FRET is effective at a distance of 1-10 nm which is equivalent to the size of macromolecules, thus providing an unprecedented level of detail on molecular interactions. The emergence of fluorescent proteins and SNAP- and CLIP- tag proteins provided FRET with the capability to monitor changes in a molecular complex in real-time making it possible to establish the functional significance of the studied molecules in a native environment. Now, FRET is widely used in biological sciences, including the field of proteomics, signal transduction, diagnostics and drug development to address questions almost unimaginable with biochemical methods and conventional microscopies. However, the underlying physics of FRET often scares biologists. Therefore, in this review, our goal is to introduce FRET to non-physicists in a lucid manner. We will also discuss our contributions to various FRET methodologies based on microscopy and flow cytometry, while describing its application for determining the molecular heterogeneity of the plasma membrane in various cell types.

MR1 presentation of vitamin B-based metabolite ligands

The major histocompatibility complex class I-related molecule MR1 can bind a novel class of antigens, namely a family of related small organic vitamin B metabolites. When bound to MR1 these metabolites are presented to a population of innate-like T cells, mucosal-associated invariant T (MAIT) cells that express a semi-invariant T cell receptor (TCR). Several non-activating and activating MR1-restricted ligands have been described, which are the degradation products of, or intermediates of, vitamin B9 (folic acid) or vitamin B2 (riboflavin), respectively. The MAIT-activating intermediates of the riboflavin synthesis pathway are unique to a wide range of microbes, and accordingly represent a molecular signature of microbial infection. Recently insights into the binding of these vitamin B metabolites to MR1, and subsequent recognition by the MAIT TCR, have been gleaned, illustrating a novel antigen presentation system.